The present invention relates to a gas sensor utilized for an air-fuel ratio control of an internal combustion engine.
A gas sensor is installed in an exhaust system of an automotive engine for air-fuel ratio control or the like.
A conventional gas sensor comprises a cylindrical insulator having an element insertion hole, a gas sensing element airtightly fixed in the element insertion hole, and a cylindrical housing having an inside space for placing the insulator. An air side cover is attached to a proximal end of the housing so as to confine an aerial atmosphere therein. And, a measured gas side cover is attached to a distal end of the housing so as to confine a measured gas atmosphere therein.
As shown in FIG. 13, to facilitate insertion of the gas sensing element 15, the element insertion hole 210 consists of a larger-diameter portion 211 formed at the proximal end thereof and a smaller-diameter portion 212 formed at the distal end thereof. The larger-diameter portion 211 has an inner diameter larger than that of the smaller-diameter portion 212.
A sealing material 219 is interposed between an outer surface of the gas sensing element 15 and an inner surface of the larger-diameter portion 211 of the element insertion hole 210 to firmly seal the clearance between them.
The clearance between the gas sensing element 15 and the element insertion hole 210 corresponds to a boundary between the aerial atmosphere and the measured gas atmosphere. It is therefore necessary to airtightly seal the clearance between the gas sensing element 15 and the element insertion hole 210 to surely separate the aerial atmosphere and the measured gas atmosphere.
However, according to the above-described conventional sensor, the smaller-diameter portion 212 is spaced from the gas sensing element 15 via a significant clearance. The gas sensing element 15 is supported at its proximal end with the sealing material 219 in a cantilever fashion.
Accordingly, when a large shock or vibration is applied from the outside, the gas sensing element will swing like a pendulum and may collide with the inner surface of the smaller-diameter portion 212. Thus, the has sensing element is often subjected to a concentrated stress and broken or damaged.
FIG. 13 shows a portion xe2x80x9cAxe2x80x9d where the concentrated stress acts to the gas sensing element 15 and a portion xe2x80x9cBxe2x80x9d where an edged portion of the insulator 21 faces to the gas sensing element 15. Accordingly, the gas sensing element 15 is often broken at these specific portions.
U.S. Pat. No. 5,886,248 discloses a gas sensor comprising a sealing material provided in a clearance between the gas sensing element and the insulator so as to extend from the proximal end to a distal end thereof.
However, as this sealing material is a hard substance, it has difficulty in preventing the gas sensing element from cracking or breaking when a large shock is applied from the outside, although such a rigid sealing material may be effective against swinging of the gas sensing element.
To solve the above-described problems, an object of the present invention is to provide a gas sensor which prevents the gas sensing element from cracking and breaking.
To accomplish the above and other related objects, the present invention provides a first gas sensor comprising a cylindrical insulator having an element insertion hole extending from a proximal end to a distal end thereof, a gas sensing element airtightly fixed in the element insertion hole of the insulator, and a cylindrical housing having an inside space for placing the insulator, with an air side cover attached to a proximal end of the housing so as to confine an aerial atmosphere therein and a measured gas side cover attached to a distal end of the housing so as to confine a measured gas atmosphere therein. According to the first gas sensor, a sealing material is provided at one side of the element insertion hole for sealing a clearance between an inner surface of the element insertion hole and an outer surface of the gas sensing element. And, a cushion filler, having the capability of withstanding a loading force ranging from 5N to 1,000N, is provided at the other side of the element insertion hole for sealing a clearance between the inner surface of the element insertion hole and the outer surface of the gas sensing element.
The present invention is characterized in that one end of the gas sensing element is fixed with the sealing material and the other end of the gas sensing element is supported by a soft cushion filler having the strength within the above-described range.
Effects of the present invention will be explained hereinafter.
The cushion filler of the present invention is so soft that it can sufficiently absorb shocks applied from the outside.
Thus, it becomes possible to prevent the gas sensing element from being directly subjected to shocks transmitted from the outside of the gas sensor.
Furthermore, as the gas sensing element of the present invention is held at both of its proximal end and its distal end, the gas sensing element does not swing like a pendulum when it receives shocks or vibrations.
Accordingly, the present invention prevents a concentrated stress from acting on a portion serving as a swing center of the gas sensing element (i.e., a portion immediately below the portion firmly fixed with the sealing material) and also prevents the gas sensing element from colliding with the inner surface of the insulator. Therefore, the present invention effectively prevents the gas sensing element from cracking or breaking.
If the loading force of the cushion filler is less than 5N, insertion of the cushion filler will be difficult. If the loading force of the cushion filler is larger than 1,000N, a large concentrated stress responsive to an external shock will act on the gas sensing element via the cushion filler. This external shock may crack or break the gas sensing element.
In view of facilitating insertion of the cushion filler, it is preferable that the loading force of the cushion filler is equal to or larger than 20N.
Furthermore, the cushion filler can effectively absorb the shock applied from the outside when the loading force of the cushion filler is equal to or less then 400N.
Furthermore, according to the present invention, the clearance between the insulator and the proximal end of the gas sensing element is filled with a dense and hard substance, such as the sealing material. As described previously, the clearance between the gas sensing element and the element insertion hole corresponds to the boundary between the aerial atmosphere and the measured gas atmosphere. Thus, the sealing material interposed between the gas sensing element and the element insertion hole surely separates the aerial atmosphere and the measured gas atmosphere.
As described above, the present invention can provide a gas sensor capable of preventing cracks and breakage of the gas sensing element.
Furthermore, as described later in a preferred embodiment of the present invention, the present invention is applicable to a gas sensor incorporating a multilayered flat plate sensing element and is also applicable to a gas sensor incorporating a cup-shaped solid electrolytic sensing element.
Furthermore, the gas sensor of the present invention is applicable to an air-fuel ratio sensor and to an oxygen sensor installed in an exhaust system of an internal combustion engine, and is also applicable to various sensors, such as a NOx sensor, a CO sensor, and an HC sensor.
The sealing material of the present invention is, for example, glass, talc, steatite, zirconia, and alumina.
The cushion filler of the present invention is, for example, a heat-resistance substance, such as zirconia or ceramic, which has a thermal expansion coefficient similar to that of the insulator or the gas sensing element.
Especially, when the cushion filler of the present invention is used in an exhaust system described in a preferred embodiment, it is subjected to high temperature exhaust gas. Thus, it is preferable that the above-described conditions are satisfied to assure the durability in a wide temperature range from the room temperature to such high temperatures.
Talc, mullite, zirconia, steatite can be also used as the cushion filler of the present invention.
When a gas sensor is solely used in a portion where the heat-resistance is not so important, various resin materials, such as PTFE (i.e., polytetrafluorethylene), fluororubber, NBR (i.e., nitrile-butadien rubber), can be also used as the cushion filler of the present invention.
Furthermore, to fill the element insertion hole with the cushion filler, a powdered material can be hardly pushed into the element insertion hole so as to serve as the cushion filler of the present invention.
Furthermore, it is possible to prepare a slurry by kneading a powered material with a binder, and injecting the slurry into the element insertion hole, and then sintering the injected slurry.
Furthermore, a dry-hardening type adhesive can be used as the cushion filler of the present invention.
Furthermore, the effect of the present invention can be obtained even if some cavities or hollow portions exist in the cushion filler.
Next, according to the present invention, it is preferable that a filling rate of the cushion filler provided between the inner surface of the element insertion hole and the outer surface of the gas sensing element is in the range from 10% to 80%.
This makes it possible to improve the strength of the cushion filler against the shock applied from the outside.
If the filling rate of the cushion filler is less than 10%, the amount of the cushion filler will be insufficient for surely holding the gas sensing element.
If the filling rate of the cushion filler is larger than 80%, the effect of absorbing the shock will be weakened and the gas sensing element may cause crack or breakage.
Next, according to the present invention, it is preferable that an injection port is provided near an open edge of the element insertion hole at the distal end of the insulator for facilitating a filling operation of the sealing material or the cushion filler.
With this arrangement, the sealing material or the cushion filler can be easily injected into the element insertion hole.
The injection port is, for example, a recess formed along the open edge (refer to FIG. 4B).
Next, according to the present invention, it is preferable that the element insertion hole comprises a larger-diameter portion at one end and a smaller-diameter portion at the opposite end, and an inner diameter of the larger-diameter portion is larger than that of the smaller-diameter portion.
With this arrangement, the strength against the external shock can be improved.
Next, according to the present invention, it is preferable that the sealing material and/or the cushion filler is placed so as to fix at least two opposed surfaces of the inner surface of the element insertion hole and the outer surface of the gas sensing element.
With this arrangement, the strength against the external shock can be enhanced.
The present invention further provides a second gas sensor comprising a cylindrical insulator having an element insertion hole extending from a proximal end to a distal end thereof, a gas sensing element airtightly fixed in the element insertion hole of the insulator, and a cylindrical housing having an inside space for placing the insulator, with an air side cover attached to a proximal end of the housing so as to confine an aerial atmosphere therein and a measured gas side cover attached to a distal end of the housing so as to confine a measured gas atmosphere therein. According to the second gas sensor, a sealing material is provided at one side of the element insertion hole for sealing a clearance between an inner surface of the element insertion hole and an outer surface of the gas sensing element. A cushion filler, having the capability of withstanding a loading force ranging from 5N to 1,000N, is provided at the other side of the element insertion hole for sealing a clearance between the inner surface of the element insertion hole and the outer surface of the gas sensing element. The insulator constitutes a main body and a separate body attached via a spacer to a distal end of the main body, so that the element insertion hole extends across both of the main body and the separate body. And, the cushion filler is provided only in the element insertion hole of the separate body.
The cushion filler of the present invention is so soft that it can sufficiently absorb shocks applied from the outside.
Thus, it becomes possible to prevent the gas sensing element from being directly subjected to shocks transmitted from the outside of the gas sensor.
Furthermore, as the gas sensing element of the present invention is held at both of its proximal end and its distal end, the gas sensing element does not swing like a pendulum when it receives shocks or vibrations.
Accordingly, the present invention prevents a concentrated stress from acting on a portion serving as a swing center of the gas sensing element (i.e., a portion immediately below the portion firmly fixed with the sealing material) and also prevents the gas sensing element from colliding with the inner surface of the insulator. Therefore, the present invention effectively prevents the gas sensing element from cracking or breaking.
If the loading force of the cushion filler is less than 5N, insertion of the cushion filler will be difficult. If the loading force of the cushion filler is larger than 1,000N, a large concentrated stress responsive to an external shock will act on the gas sensing element via the cushion filler. This external shock may crack or break the gas sensing element.
Furthermore, according to the present invention, the clearance between the insulator and the proximal end of the gas sensing element is filled with a dense and hard substance, such as the sealing material. As described previously, the clearance between the gas sensing element and the element insertion hole corresponds to the boundary between the aerial atmosphere and the measured gas atmosphere. Thus, the sealing material interposed between the gas sensing element and the element insertion hole surely separates the aerial atmosphere and the measured gas atmosphere.
Furthermore, as the insulator constitutes the main body and the separate body attached via the spacer to the distal end of the main body, the external shock can be effectively absorbed by the spacer. This enhances the strength against the external shock.
Furthermore, as the cushion material is solely provided at the separate body, the filling operation of the cushion filler can be easily performed.
As described above, the present invention can provide a gas sensor capable of preventing cracks and breakage of the gas sensing element.